An example of a measuring method and a related measuring device of this general type is disclosed in an article in Optical EngineeringVol. 24 (1985), page 966. In that disclosure, a pattern of narrow light stripes and large dark, intermediate spaces is projected on the object to be measured. The grating pattern, which is deformed on the object surface, is recorded with two cameras; and the deformation of the brightness pattern evaluated. A second camera assures that reference coordinates and the object are measured in chronological order. To prevent synchronization problems during camera readout, both cameras project an image of the object on the same camera target. The disadvantage of this prior art arrangement is that the object surface is measured only along the light stripes between which are relatively wide intermediate spaces where no measurement takes place. Therefore, in order to capture the topography of the object surface completely, eight measurements must be performed successively with shifted patterns. Furthermore, the relative measuring resolution is relatively low, namely, being 50/1 (measuring range/measuring resolution) perpendicular to the sample surface.
U.S. Pat. Nos. 4,641,972 and 4,488,172 disclose so-called grating projection systems. In these disclosures, sinusoidal brightness patterns are projected on the object surface and the patterns are recorded with a single camera. Three or four images are recorded in chronological order, whereby the pattern is shifted by one-third or one-fourth of a grating period between the recording of images. The grating patterns are evaluated by means of algorithms known from phase shift interferometry, whereby the grating phase can be determined to approximately 2* .pi./100. The disadvantage of these prior art systems is that the phase evaluation produces unambiguous measured results only within one grating order. To solve this problem, U.S. Pat. No. 4,641,972 uses so-called phase unwrapping which has also been known from interferometry. Phase unwrapping requires that the surface be continuous and a starting point be known. When the measured values are noisy, this method may compute an erroneous measured value and then continue in an erroneous manner so that large-area errors occur in the measured result. This method fails entirely when the measured object is discontinuous.
To increase the range over which the measurement is unambiguous, U.S. Pat. No. 4,488,172 has disclosed the projection of two grating patterns whereby one grating pattern has eight times the period width of the other grating pattern. When the patterns are phase-shifted, however, they move at different speeds or by different distances perpendicular to the direction of the projected stripe. As a result of this, the relative order of the grating pattern with respect to the coordinate system of the equipment may easily be lost.
EP-OS 0,076,866 discloses the simultaneous projection of three color-coded patterns on the object, whereby the patterns are phase-shifted by 120 degrees relative to each other. The grating patterns deformed by the object surface are recorded corresponding to their color coding by three associated color-selective cameras. As a result of this, the pattern is recorded simultaneously in three different phase relations so that an evaluation based on the phase shift algorithm may be performed without requiring a mechanical shifting. However, the use of three cameras does not accomplish an expansion of the unambiguous range.
UK Patent Application No. 2,204,397A discloses a measuring system where three projector/camera systems are positioned at an angle of 120 degrees relative to each other around the center of the measured volume. Each camera records an image of the pattern which is generated by the projector. The use of three projector/camera systems permits the measurement of all sample surfaces of an object positioned in a measured volume. This system also does not result in an expansion of the unambiguous range in the direction of the optical axes of the projectors.
U.S. Pat. No. 4,937,445 discloses apparatus with which grating patterns are projected on the surface of the object and then recorded with two CCD cameras. Both CCD cameras are positioned at the same distance from the center of projection of the grating projector and record images of different grating patterns projected on the object. Two linear image sensors are used only to divide in half the number of lines projected on each image sensor. The evaluation by the image sensors is analogous to the above-cited Optical Engineering reference, in that the lateral position of the projected stripes is measured. In this case, grating phases are not computed.
Further, PCT International Application Publication WO 91/09273 discloses a photogrammetric measuring device whereby three cameras record images of the measured object and at the same time a reference pattern is superimposed on the images of the measured object. However, this recent prior art system does not project a grating pattern on the object, nor does it compute fringe phases based on the video images.
The present invention is a measuring system of the type identified above, and its objects are to accomplish the desired three-dimensional measurement of object surfaces with (a) less complex apparatus, (b) a high depth resolution, and (c) an expanded range of unambiguous measurement. The methods and apparatus of the invention are also intended to permit the measurement of discontinuous object surfaces.